In electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged, and exposed to a light pattern of an original image to be reproduced to selectively discharge the surface in accordance therewith. The resulting pattern of charged and discharaged areas on that surface form an electrostatic charge pattern (an electrostatic latent image) conforming to the original image. The latent image is developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner". Toner is held on the image areas by the electrostatic charge on the surface. Thus, a toner image is produced in conformity with a light image of the original being reproduced. The toner image may then be transferred to a substrate (e.g., paper), and the image affixed thereto to form a permanent record of the image to be reproduced. Subsequent to development, excess toner left on the charge retentive surface is cleaned from the surface. The process is well known, and useful for light lens copying from an original, and printing applications from electronically generated or stored originals, where a charged surface may be imagewise discharged in a variety of ways.
It is common practice in electrophotography to use corona generating devices to provide electrostatic fields driving various machine operations. Thus, corona devices are used to deposit charge on the charge retentive surface or photoreceptor prior to exposure to light, to implement toner transfer from the photoreceptor surface to the substrate, to neutralize charge on the substrate for removal from the photoreceptor surface, and to assist cleaning of the photoreceptor surface after toner has been transferred to the substrate. These corona devices normally incorporate at least one coronode held at a high voltage to generate ions or charging current to charge a surface closely adjacent to the device to a uniform voltage potential, and may contain screens and other auxiliary coronodes to regulate the charging current or control the uniformity of charge deposited.
A number of corona devices are driven with an A.C. voltage potential. For example, in the Xerox 9500 duplicator, A.C. driven corotrons, corona charging devices comprising a bare wire coronode held between insulating end blocks and surrounded by a conductive housing usually held at a ground potential, are used at pre-transfer, detack, and pre-clean stations.
Dicorotrons, A.C. driven corona charging devices provided with a dielectric coated coronode and a control electrode or shield, are noted for the capability of charging a surface to the potential of the shield. In the Xerox 1090 copier, a tandem pair of dicorotrons operate to charge the photoreceptor surface preparatory to exposure. In that arrangement, a first dicorotron charges the surface with a known A.C. voltage applied to the coronode and a known D.C. voltage applied to the shield. A second dicorotron is also driven with an A.C. voltage and a shield voltage. Any difference between the surface voltage, as applied by the first dicorotron, and the shield voltage of the second dicorotron causes a shield current in the second dicorotron which is related to the difference. This shield current is useful in a feedback arrangement that may be used to control the first dicorotron shield voltage. U.S. Pat. No. 4,456,370 to Hayes, Jr. demonstrates a feedback arrangement using such a combination.
Scorotrons, which may be A.C. driven corona charging devices, are characterized by a conductive screen or grid interposed between a coronode and photoreceptor surface, and held at a voltage corresponding to the desired charge on the photoreceptor surface. A D.C. voltage is applied to the scorotron grid. The grid tends to share the corona current with the photoreceptor surface. As the voltage on the photoreceptor surface increases towards the voltage level of the grid, corona current flow to the grid is increased, until all the the corona current flows to the grid and no further charging of the photoreceptor surface takes place. U.S. Pat. No. 4,074,134 to Roalson appears to show a scorotron charging device, D.C. in this case, where the voltage on the screen is used to derive a signal for comparison to a reference to control the high voltage corona power source. JP-A No. 59-228678 shows a similar arrangement.
In U.S. Pat. No. 3,604,925 to Snelling et al., an arrangement is described for a D.C. corotron in which a bare wire electrode is positioned adjacent the device to detect a portion of the corona curret (i.sub.s) attracted to the shield. The current detected by the bare wire electrode is related to the surface potential (V.sub.p) potential on the charged surface. However, this relationship is only linear over a relatively small voltage range, and thus provides only limited utility. Similar arrangements are shown by JP-A No. 60-107051, and U.S. Pat. No. 4,431,302 to Weber.
In the Xerox 9500 copier, photoreceptor residual potential cycle-up has the effect of changing the development/cleaning fields, and thus the copying characteristics. The magnitude of these fields depends upon the relationship between photoreceptor surface potential and developer roll bias. It would be highly desirable to measure the photoreceptor surface potential to derive a feedback control of the developer roll bias to maintain a constant relationship between the development/cleaning fields and photoreceptor.